Dear Xavi,

Thanks a lot for the prompt reply and the detailed address of the points I’ve 
outlined. 

For completeness, see inline.


> Le 1 mars 2018 à 12:50, Xavi Vilajosana Guillen <xvilajos...@uoc.edu> a écrit 
> :
> 
> Dear Alex,
> 
> Thanks so much for your constructive review. Let us answer inline your 
> comments (XV:). We are taking them into account in the new draft version that 
> will be published before the cut-off date.
> 
> regards
> Xavi
> ----
> Reviewer: Alexander Pelov
> Review result: Ready with Issues
> 
> Hello all,
> 
> This is the review for the IoT Directorate.
> 
> Document: draft-ietf-6tisch-6top-protocol-09
> Reviewer: Alexander Pelov
> Date: 22 February 2018
> 
> The general feeling of the reviewer is that the document is a solid work.
> Multiple examples are given and the document is easy to understand as a whole.
> There are some nits, and some text that need to be clarifier.
> 
> The general feeling of the reviewer is that the document relies heavily on the
> definition of an external Scheduling Function (SF). The recommended values 
> seem
> very reasonable to the reviewer and it is not clear what is the benefit of
> anticipating that an SF can override the semantics of most of the fields. For
> example, most of the fields are opaque to the 6P sublayer and only make sense
> to the SF : CellOptions, Metadata, CellList. For one, in Wireshark, there will
> be the need for separate disector for each SF.
> 
> XV: we aimed to support particular needs of an SF. For example the Metadata 
> field can be used to indicate to what Slotframe Handle the 6P operation 
> should be applied. However we think as well that a large set of SFs will use 
> the fields as defined by 6P (celllist) for example.

Ok. Thanks for the example with the Metadata field; 

> 
> A final point here is that there seem to be no readily available polished SF
> that would help in the understanding of the concepts beyond what is already on
> the 6P draft. 
> 
> XV: We think the MSF (https://tools.ietf.org/html/draft-chang-6tisch-msf-00 
> <https://tools.ietf.org/html/draft-chang-6tisch-msf-00>) clearly maps to the 
> requirements from 6P.

Thanks for clearing this out. If MSF becomes a WG item I would find it useful 
to add it to the text as a reference (that’s a very minor edit and shouldn’t 
interfere with the closing of the WGLC).

> 
> 
>  For example, the SF0 draft (draft-ietf-6tisch-6top-sfx-00)
> redefines quite heavily the behavior of CellList introducing notion of
> WhiteList and BlackList of cells. The reviewer is aware that these are 
> distinct
> works, but it feels that there should be a minimum level of interoperability,
> where an upper layer does not completely redefine what is happening on lower
> layers. Having extension mechanisms may seem like a better way to solve
> richness of proposals if this is necessary.
> 
> XV: We agree on that. We think however that most SFs can be developed without 
> redefining the 6P fields. Note also that the SIGNAL command is designed to 
> that aim. i.e., an SF issues SIGNALS which are opaque to 6P internals in 
> order to transmit information to the other Node SF.

Ok.

> 
> One point which remained unclear is how do the Minimal 6TiSCH and 6P interact?
> It could be useful to provide a description on the bootstrap of 6P interaction
> (how does a sender A initiate the first 6P Request - over Minimal
> 6TiSCH-managed cell?).
> 
> XV: This is detailed in the MSF draft for example. 6P defines the messaging 
> structure and protocol interaction but does not define a particular behaviour.
> The SF is the responsible of defining the behaviour at boot, what cells are 
> used and how new cells are added. 6P Provides the l2 transport semantics for 
> the 
> SF to operate. 

Ok, thanks. One more reason to add reference to MSF.

> 
>  
> How do they enter in play in case of de-synchronisation
> ? (e.g. A rescheduling all 6P cells, but B not getting the final L2 ACK, which
> puts A's 6P cells on a completely different schedule than B's.. so B can't
> signal back transaction rollback / CLEAR). Is this solved by 6TiSCH minimal or
> through a different mechanism?

I think that independently of MSF, this is a situation that could apply to any 
SF. It may then be of interest to describe in the text of 6P how is this 
situation handled. (even if MSF handles this gracefully, other SFs may benefit 
from guidance how to handle such situation).

> 
> The Security section could be enriched. A notable example is the handling of
> resource reservation, which could lead to DOS attacks.
> 
> XV: this has been clarified thanks to another reviewer comment. 

Ok, thanks.

> - - - - - - - - - - - - - - -
> 
> Section 3.2.3:
> 6P CellOptions - ends with the statement that it is an "Opaque set of bits",
> which MAY be redefined by the SF (format, meaning). As pointed out earlier, if
> there is a need to redefine this for each SF, maybe there are other ways of
> defining such flexibility (e.g. TLVs).
> 
> XV: We enable an SF to redefine that field but we do not expect that most of 
> the SFs redefine it. 

If you feel this could be a blocking issue down the road, then I’m OK with 
leaving it as is. 

> 
> The table in Figure 7 provides the recommended meaning of the bitmap for 6P
> COUNT and 6P LIST. What is the recommended meaning for 6P ADD/DELETE/RELOCATE?
> 
> XV: thanks, we clarify this in the text. We added Figure 8 with a table 
> describing the 
> behaviour of 6P when the different cellOPtions are present in 
> ADD/DELETE/RELOCATE requests.
> 

Ok, thanks.

> Nits: there seems to be errors in Figure 7: examples of "all cells are marked
> as RX" and "all cells are marked as TX" seem inverted (same for TX=1,RX=0,S=1
> and TX=0,RX=1,S=1).
> 
> XV: no, this is correct :). The request is issued by node A, saying for 
> example COUNT TX cells to B. B responds with the list of cells marked as RX 
> in its schedule for neighbor A. (as in A they are marked as TX).
> 

Oh, I see. Adding a sentence to refresh the memory of the people like me would 
help here ;)

> - - - - - - - - - - - - - - -
> 
> Section 3.3.1:
> How does the sender/receiver know the size of CellList? (infer from packet
> size?)
> 
> XV: the IE header contains the size of the 6top IE. The header field sizes 
> are known and hence the celllist length can be determined from that. 
> 

Can you please add a clarification to the text for this? 

> 
> The candidate cells (a total of NumCandidate) are presumably provided by the
> SF. However, it is up to 6P to handle the case when they do not fit in the
> packet size. The text specifies that this should be handled in more than one 
> 6P
> ADD requests - which is OK on the conceptual level, but seems underspecified
> for an implementation. 
> 
> What if NumCells is smaller than the number of candidate
> cells that can fit in a single transaction - should they be also split in two
> transactions?
> 
> XV: NumCells tells how many cells need to be added/deleted/relocated. I think 
> you refer to the 6P list command instead. 
> In a 6P LIST Command we use MaxNumCells which indicate an upper bound of the 
> cells to be listed (lique in SQL when we do LIMIT).
> If in a 6p LIST the number of returned cells is smaller than MaxNumCells, 
> then the issuer may send another LIST with an specific 
> offset (e.g the number of cells received) in order to get the remaining 
> cells. 
> This is how "pagination works" indeed.

No, I meant the 6P ADD case, but I was thinking about the 3-step 6P 
transaction. When B returns the list of candidate cells, how is that handled in 
case it cannot fit the 6P packet? (again, it can be trivial)

> 
> 
>  What happens if the first 6P ADD is successful, but the second
> one fails? Should the sender 6P DELETE the successfully added first batch of
> cells?
> 
> XV:If you refer to an ADD Request where numcells is larger than the number of 
> cells that fit in a packet, then this should be handled by the SF, splitting 
> the request
> in multiple ADD operations. If one fails a node can retray later.  

Ok, thanks.

> 
> 
> Can allocation of 0 cells be considered as partial success?
> 
> NOALLOC return code is not defined.
> 
> XV: Why someone wants to do a 0 cell ADD request? I think that the response 
> in this case can be RC_SUCCESS, indicating that 0 cells have been scheduled.
> 

"The returned list can contain NumCells elements (succeeded) or
   between 0 and NumCells elements (partially succeeded)."

I read this as inclusive between, e.g. 0 and NumCells included. This for me 
meant that an addition of 0 elements was considered a partial success. 


> - - - - - - - - - - - - - - -
> 
> Section 3.3.3.
> Figure 17 - it seems counterintuitive to have RC_SUCCESS on failed relocation.
> Could NOALLOC be used in this case?
> 
> XV: We had long discussions about this while we wrote the specification. Our 
> consensus was that the return code indicates that the 6P transaction worked 
> (RC_SUCCESS)
> and that the celllist length tells us the result in terms of cells relocated. 
> So no strong arguments in both sides I guess but we agreed to take that 
> approach.

If you’ve already had a long discussion on this on the ML it works for me. 
Thanks.

> 
> In both Relocation and Allocation 3-step 6P transaction there is the risk of a
> security attack. If a malicious node constantly renews 3-step requests and
> never acknowledges, the neighboring node will be keeping the proposed cells as
> "reserved" and not allocate them to other nodes, thus provoding a DOS attack.
> Probably a way to limit repeated requests could be useful for this case.
> 
> XV: We know that any 3-step transaction/protocol can be subject to a DoS 
> attack as long as one of the messages is 
> not replayed (same happens with TCP handshake attacks). We do not want to 
> introduce policies to handle that for a particular 
> situation but we think that this needs to be clarified in the security 
> considerations section. 
> To this aim we indicated the following: 
> We added a consideration in the security section. 
> 
> The 6P protocol does not provide protection against DOS attacks. This is 
> relevant in 3-step transactions when a confirmation message could not be sent
> in purpose by the attacker. Such situations SHOULD be handled by an 
> appropiate policy such as blacklisting the attacker after several attempts.
> Other DoS attacks are possible by sending unmeaningful requests to nodes. The 
> effect to the overall network can be minimal as communication between 
> attacked node
> and attacker happen in dedicated cells. DoS then only limits that cells. Yet, 
> this can be avoided by blacklisting the node after several attempts. When to 
> blacklist 
> is policy specific and SHOULD be addressed by the SF. 
> 

Perfect, thanks !

> - - - - - - - - - - - - - - -
> 
> Section 3.3.5.
> "To retrieve the list of scheduled cells at B" - all cells scheduled at B? Or
> the cells scheduled for A? (could be clarified)
> 
> XV: We rephrased like this:
>  To retrieve a list of scheduled cells at B, node A issues a 6P LIST command.

Ok, sounds good. Can you add a sentence of the sort: "This list is only limited 
to the cells scheduled for A" (if this is true) ?

>  
> 
> Nits: Node B MAY returns -> Node B MAY return
> XV: Thanks. we fixed that.

Ok thanks.

> 
> - - - - - - - - - - - - - - -
> 
> Section 3.3.6.
> There may be two parallel transactions: 1) A->B and 2) B->A. If a 6P CLEAR is
> issued on one, how does this affect the other? (presumably clear both?)
> 
> XV: A transaction is not applied until the transaction is committed, this is 
> the Confirmation message is received on one side and the L2 ACK is received 
> at the other side.
> In a particular slot a node may be only receiving or sending a packet at a 
> time and hence this B->A A->B transaction cannot happen unless they use 2 
> radios.
> In case of using 2 radios this may lead to an inconsistency in the schedules 
> that will be resolved in the next message thanks to the SeqNum set to 0 in 
> the side that cleared.

If both A and B start 3-step transaction, wouldn’t that allow for parallel 
transactions to exist, even in single radio? Also, is the use of 2 radios 
excluded? 
The way I understand your answer, there is no explicit influence from one 
transaction to the other - it’s up to the SeqNum to settle the situation.

Works fine for me and is simple, thanks for clarifying.

>  
> How does this affect separate SF? If there is a state kept by each SF, are all
> SFs cleared? Are statistics also cleared for SFs? (probably SF-dependent, out
> of the scope)
> 
> XV: the commands use an SFID that maps the action to a particular SF. Hence 
> if a clear happens in the cells scheduled by one SF other cells scheduled by 
> another SF won't be affected.

Oh, OK, so now I have another question - how are the SeqNum handled across SF? 
Is there a SeqNum counter PER SF? 
By reading the text, "That is, a node stores as many SeqNum values as it has 
neighbors." - that would imply that the SeqNum counter is shared by the SF 
(e.g. it is on the 6P level, which seems consistent to me).

> 
> - - - - - - - - - - - - - - -
> 
> Section 3.4.6.
> Figure 27: "Clear or Reset" - Reset could be ambiguous (device has restarted 
> vs
> transaction failed, RC_RESET)
> 
> XV: We clarified with:
> Clear or After device Reset
> 

Ok, thanks.

> - - - - - - - - - - - - - - -
> 
> Section 6.2.5.
> Consider having Specification required for the range SFID 128-255.
> XV: Expert review is a well understood term.

Yes, so is Specification required. Has this policy been discussed on the ML? 



> 
> - - - - - - - - - - - - - - -
> 
> Section 6.2.4.
> It would have seem more readable to have RC_ERR_ prefix for errors. It may not
> be outright evident that RC_CELLLIST or RC_VERSION is an error.
> 
> XV:Thanks for this comment. We renamed them as indicated.

Ok, thanks.

> - - - - - - - - - - - - - - -
> 
> Overall a rich document, with probably some minor changes to be made.
> 
> Best,
> Alexander

Thanks again, Xavi, for the prompt reply and for making the necessary changes!
Alex



> 
> 
> 2018-02-23 1:35 GMT+01:00 Alexander Pelov <a...@ackl.io 
> <mailto:a...@ackl.io>>:
> Reviewer: Alexander Pelov
> Review result: Ready with Issues
> 
> Hello all,
> 
> This is the review for the IoT Directorate.
> 
> Document: draft-ietf-6tisch-6top-protocol-09
> Reviewer: Alexander Pelov
> Date: 22 February 2018
> 
> The general feeling of the reviewer is that the document is a solid work.
> Multiple examples are given and the document is easy to understand as a whole.
> There are some nits, and some text that need to be clarifier.
> 
> The general feeling of the reviewer is that the document relies heavily on the
> definition of an external Scheduling Function (SF). The recommended values 
> seem
> very reasonable to the reviewer and it is not clear what is the benefit of
> anticipating that an SF can override the semantics of most of the fields. For
> example, most of the fields are opaque to the 6P sublayer and only make sense
> to the SF : CellOptions, Metadata, CellList. For one, in Wireshark, there will
> be the need for separate disector for each SF.
> 
> A final point here is that there seem to be no readily available polished SF
> that would help in the understanding of the concepts beyond what is already on
> the 6P draft.  For example, the SF0 draft (draft-ietf-6tisch-6top-sfx-00)
> redefines quite heavily the behavior of CellList introducing notion of
> WhiteList and BlackList of cells. The reviewer is aware that these are 
> distinct
> works, but it feels that there should be a minimum level of interoperability,
> where an upper layer does not completely redefine what is happening on lower
> layers. Having extension mechanisms may seem like a better way to solve
> richness of proposals if this is necessary.
> 
> One point which remained unclear is how do the Minimal 6TiSCH and 6P interact?
> It could be useful to provide a description on the bootstrap of 6P interaction
> (how does a sender A initiate the first 6P Request - over Minimal
> 6TiSCH-managed cell?). How do they enter in play in case of de-synchronisation
> ? (e.g. A rescheduling all 6P cells, but B not getting the final L2 ACK, which
> puts A's 6P cells on a completely different schedule than B's.. so B can't
> signal back transaction rollback / CLEAR). Is this solved by 6TiSCH minimal or
> through a different mechanism?
> 
> The Security section could be enriched. A notable example is the handling of
> resource reservation, which could lead to DOS attacks.
> 
> - - - - - - - - - - - - - - -
> 
> Section 3.2.3:
> 6P CellOptions - ends with the statement that it is an "Opaque set of bits",
> which MAY be redefined by the SF (format, meaning). As pointed out earlier, if
> there is a need to redefine this for each SF, maybe there are other ways of
> defining such flexibility (e.g. TLVs).
> 
> The table in Figure 7 provides the recommended meaning of the bitmap for 6P
> COUNT and 6P LIST. What is the recommended meaning for 6P ADD/DELETE/RELOCATE?
> 
> Nits: there seems to be errors in Figure 7: examples of "all cells are marked
> as RX" and "all cells are marked as TX" seem inverted (same for TX=1,RX=0,S=1
> and TX=0,RX=1,S=1).
> 
> - - - - - - - - - - - - - - -
> 
> Section 3.3.1:
> How does the sender/receiver know the size of CellList? (infer from packet
> size?)
> 
> The candidate cells (a total of NumCandidate) are presumably provided by the
> SF. However, it is up to 6P to handle the case when they do not fit in the
> packet size. The text specifies that this should be handled in more than one 
> 6P
> ADD requests - which is OK on the conceptual level, but seems underspecified
> for an implementation. What if NumCells is smaller than the number of 
> candidate
> cells that can fit in a single transaction - should they be also split in two
> transactions? What happens if the first 6P ADD is successful, but the second
> one fails? Should the sender 6P DELETE the successfully added first batch of
> cells?
> 
> Can allocation of 0 cells be considered as partial success?
> 
> NOALLOC return code is not defined.
> 
> - - - - - - - - - - - - - - -
> 
> Section 3.3.3.
> Figure 17 - it seems counterintuitive to have RC_SUCCESS on failed relocation.
> Could NOALLOC be used in this case?
> 
> In both Relocation and Allocation 3-step 6P transaction there is the risk of a
> security attack. If a malicious node constantly renews 3-step requests and
> never acknowledges, the neighboring node will be keeping the proposed cells as
> "reserved" and not allocate them to other nodes, thus provoding a DOS attack.
> Probably a way to limit repeated requests could be useful for this case.
> 
> - - - - - - - - - - - - - - -
> 
> Section 3.3.5.
> "To retrieve the list of scheduled cells at B" - all cells scheduled at B? Or
> the cells scheduled for A? (could be clarified)
> 
> Nits: Node B MAY returns -> Node B MAY return
> 
> - - - - - - - - - - - - - - -
> 
> Section 3.3.6.
> There may be two parallel transactions: 1) A->B and 2) B->A. If a 6P CLEAR is
> issued on one, how does this affect the other? (presumably clear both?)
> 
> How does this affect separate SF? If there is a state kept by each SF, are all
> SFs cleared? Are statistics also cleared for SFs? (probably SF-dependent, out
> of the scope)
> 
> - - - - - - - - - - - - - - -
> 
> Section 3.4.6.
> Figure 27: "Clear or Reset" - Reset could be ambiguous (device has restarted 
> vs
> transaction failed, RC_RESET)
> 
> - - - - - - - - - - - - - - -
> 
> Section 6.2.5.
> Consider having Specification required for the range SFID 128-255.
> 
> - - - - - - - - - - - - - - -
> 
> Section 6.2.4.
> It would have seem more readable to have RC_ERR_ prefix for errors. It may not
> be outright evident that RC_CELLLIST or RC_VERSION is an error.
> 
> - - - - - - - - - - - - - - -
> 
> Overall a rich document, with probably some minor changes to be made.
> 
> Best,
> Alexander
> 
> 
> _______________________________________________
> 6tisch mailing list
> 6tisch@ietf.org <mailto:6tisch@ietf.org>
> https://www.ietf.org/mailman/listinfo/6tisch 
> <https://www.ietf.org/mailman/listinfo/6tisch>
> 
> 
> 
> -- 
> Dr. Xavier Vilajosana
> Wireless Networks Lab
> Internet Interdisciplinary Institute (IN3)
> Professor
> (+34) 646 633 681
> xvilajos...@uoc.edu <mailto:usu...@uoc.edu>
> http://xvilajosana.org <http://xvilajosana.org/>
> http://wine.rdi.uoc.edu <http://wine.rdi.uoc.edu/>
> Parc Mediterrani de la Tecnologia 
> Av Carl Friedrich Gauss 5, B3 Building
> 08860 Castelldefels (Barcelona). Catalonia. Spain
>   
> _______________________________________________
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> iot-...@ietf.org
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